The present invention relates to non-volatile semiconductor memories.
Such semiconductor memories as flash memory and EEPROM are non-volatile and electrically rewritable. Accordingly, these semiconductor memories are widely used as program-used or data-used memories in apparatuses such as digital household electrical-appliances and vehicle-installed controllers. In the conventional products of these non-volatile semiconductor memories, the high-speed implementation and large-capacity implementation require miniaturization of the elements. It has been known, however, that, in order to maintain the data retention characteristics, a limit exists in the thinning of a silicon oxide film used as the tunnel insulating film in particular. Also, a degradation occurs in the tunnel insulating film, because hot carriers are injected into a floating gate via the tunnel insulating film at the time of the rewriting. Consequently, in the status quo, the thinning of the tunnel insulating film is getting closer to its limit.
In order to solve the above-described problem, researches and developments are now being performed concerning non-volatile memories of various types of novel schemes. As one of such novel non-volatile memories, there exists a silicon nano-dots memory where polycrystalline silicon is formed in a dots-like manner instead of a polycrystalline silicon film of the floating gate. This silicon nano-dots memory accumulates electrons in the discrete dots. Accordingly, even if a leakage path is formed within the tunnel oxide film due to the rewriting, only electrons accumulated in a part of the dots pass therethrough. As a result, the silicon nano-dots memory is expected to be highly reliable. Also, it is expected as well to cause multi-bit information to be memorized into a single memory cell by taking advantage of a difference between threshold voltages due to a difference between writing areas. This is performed by selectively writing electrons into a part of dots (S. Tiwari et al.: IEEE International Electron Devices Meeting, pp. 521-524 (1995)).
In the silicon nano-dots memory, however, there occurs a problem of the element-characteristics variation which accompanies nonuniformity of the dots. Consequently, the uniform dot formation is requested. Also, even if rewriting resistance is enhanced, the silicon oxide film is used as the tunnel insulating film. As long as this fact remains unchanged, this enhancement cannot be said to be an essential solution for conquering the limit to the thinning of the tunnel insulating film thickness for maintaining the data retention characteristics.